1. Field of the Invention
This invention relates to a fluorescent X-ray analyzing apparatus for conducting element analysis by detecting a secondarily-produced X-ray emanating from a sample in response to illuminating an X-ray onto the sample.
2. Description of the Prior Art
There are broad uses of the fluorescent X-ray analyzing apparatus for conducting element analysis by illuminating a primary X-ray onto a sample and detecting an X-ray secondarily produced from the sample in response thereto.
Particularly, there is a proposal of a method in which, in order to measure an element contained in slight amount contained in a solution, approximately 50 micro-liters of a solution is dripped onto an extremely-thin organic thin film through a micropipette and then dried so that the xe2x80x9cresiduexe2x80x9d on the organic thin film can be thereafter measured by a fluorescent X-ray analyzing apparatus (called an organic thin film drip and dry method) (disclosed in 34th X-ray analysis forum, on Nov. 10, 1998). The principle of this measurement is briefly explained.
Referring to FIG. 2, there is illustrated a schematic view where measurement is conducted with a conventionally-used fluorescent X-ray analyzing apparatus.
Numeral 1 is a sample stage and numeral 3 is an organic thin film support member.
Further, numeral 4 is a residue and numeral 5 is an organic thin film.
Furthermore, numeral 10 is a X-ray source and numeral 11 is a detector.
When illuminating a primary x-ray to a sample, an X-ray spectrum obtained by a detector includes (1) a fluorescent X-ray due to an element contained in the sample and (2) a component of the primary X-ray reaching the detector due to scattering of the primary X-ray (includes diffraction) upon the sample. The scattering X-ray might form a broad background over a wide energy range of the spectrum. In the meanwhile, because the scattering X-ray decreases in intensity with a decrease in sample thickness, it is possible to obtain a spectrum with a low background.
The relationship between a detection lower limit and a background intensity can be defined as follows:
(X-ray intensity obtained from element concentration at detection lower limit)=3xc3x97 (background X-ray intensity).
Accordingly, the use of the organic thin film dip-and-dry method greatly improves a lower limit of detection as compared to the method of analysis that directly illuminates an X-ray to a solution sample.
In the meanwhile, in the fluorescent X-ray analyzing apparatus for measuring an organic thin film formed through dripping and drying a sample, a top-radiation system for illuminating a primary X-ray from the sample above is adopted so as not to spill a residue out of the organic thin film. Meanwhile, a fluorescent X-ray analysis apparatus is used to illuminate a primary X-ray only to an area of the residue. This apparatus is provided with a collimator for forming a beam as fine as about 1 to 2 mm, a sample stage for positioning a the residue, and a sample observation mechanism.
In the conventional apparatus, however, the sample stage is made of a resin or aluminum. Accordingly, when a primary X-ray is illuminated on to an organic thin film, the X-ray transmitting through the organic thin film impinges upon the sample stage. This results in a problem that the detector detect a scattering X-ray from the stage and a fluorescent X-ray thus causing a background.
In order to solve the problem, in the present invention, a hole is formed in a sample stage which has a greater diameter than that of a beam of a primary X-ray and a coating film is formed of a heavy metal on the bottom and inner surfaces of the hole. The sample stage is made lightweight to have a movement mechanism, and uses resin or light metal such as aluminum. Because the intensity of a scattering X-ray is inversely proportional to a mean atomic number of an object, the scattering X-ray from the sample stage formed of resin or light metal is extremely high in intensity. To form the entire sample stage of a heavy metal is not advantageous because of a resulting increase in weight. Accordingly, the sample stage itself is formed of a resin or a lightweight material, and a hole is formed in a position so that a primary X-ray is caused to pass therethrough. The hole at its bottom surface where the primary X-ray is to be finally stopped is made of a heavy metal. Due to this, the scattering X-ray is reduced in intensity without requiring weight increase for the entire stage. Furthermore, a heavy metal coating film is also formed on an inner surface of the hole, thereby preventing scattering at the inner surface. Meanwhile, the scattering X-ray from the heavy metal coating film, although low in intensity, possibly cause an interference peak on a spectrum. However, in the case that the coating film is formed of the same substance as an element of a target material of the X-ray tube, an effect is obtained in that the interference peak will not increase because of the fact that a characteristic X-ray peak concerning the target material element exists in nature on the spectrum.